In the treatment of patients, more particularly with liquid medications, a fixed, adjusted rate of infusion is often insufficient, and it is necessary that the amount of liquid infused per time unit has to be adjusted repeatedly at certain time intervals. Such a readjustment is particularly required if insulin is to be infused continuously in the treatment of diabetes, because the insulin requirement of the diabetic patient is subject to substantial fluctuations during the daytime, said fluctuations being caused, for example, by the cycle of meals, while it is nearly constant in time during night hours. An implantable apparatus of the above type, which primarily serves for the controlled feeding of insulin, is known, for example, from U.S. Pat. No. 3 894 538. This known device operates according to the electro-osmotic principle, i.e., the discharge or administering of insulin is controlled or regulated by the fact that particles of gas or liquid migrate in electro-osmotic manner into a first receptacle having a variable volume, which migration of said particles takes place in dependence on electrical signals applied to electrodes. As a result of the volume expansion of said receptacle, a separating diaphragm is deflected into a second receptacle serving as the receptacle which contains the medication, whereby the medication is expelled from said second receptacle in the desired amount, or dose.
Furthermore, a device is known from U.S. Pat. No. 3 837 339 in which the glucose level in the human body is continuously measured by means of an implantable glucose sensor, and a switching signal is emitted by a voltage comparator to a switching valve if said level rises beyond a predeterminable value. The switching valve so activated connects the blood circulatory system of the controlled patient with an insulin-containing receptacle which, for example, may also be implanted, whereupon insulin is fed or supplied into the blood circulatory system until the glucose level has declined or fallen back below the adjusted normal value. The comparator will thereupon no longer produce an output signal, and the valve disposed between the blood circulatory system and the insulin receptacle is closed.
Finally, it has been proposed to provide for a fixed prior programming of the entire daily administration of a medication (more particularly, insulin), comprising moving the piston of a medication injector by means of a motor drive in accordance with said program in such a way that the desired daily dose is achieved while taking into account all fluctuations.
The apparatus according to U.S. Pat. No. 3 894 538 does have the advantage of achieving a continuous dosing up to the smallest values; however, the continuous feeding or ejection of medication also requires a continuous generation of pressure, and consequently, a constantly high requirement of input energy for the controlling electrodes, which requirement increases substantially when the rate of medication infeed increases. The apparatus according to U.S. Pat. No. 3 837 339 has an energy requirement which is relatively high, and furthermore does not permit fine dosing of the medication. The output of medication takes place rather at a constantly high rate, namely in each case for the duration of a comparator signal. Since the duration of medication infeed may be long owing to the inertia of the blood circulation and the electrical control system, there is the risk of continuous overdosing, whereby the supply of medication is rapidly exhausted, and more particularly, injury to the patient cannot be excluded, (an excess of insulin may result, for example, in life-endangering hypoglycemic shock). An apparatus comprising a piston injector has relatively high inertia on account of its design and mode of operation, and accordingly it does not permit sufficiently small dosings and requiries an excessively high expenditure of energy.